The present invention relates to a sheet feeder used for an image forming apparatus such as a laser printer or an electronic copying machine.
With an image forming apparatus such as a laser printer or an electronic copying machine, cut sheets are set in a sheet containing unit such as a sheet feed cassette or sheet feed tray (hereinafter referred to as "sheet feed cassette"), and the sheet feed cassette is installed in a sheet feeder of the image forming apparatus, so that sheets can be fed one by one using a sheet feed member such as a sheet feed roller arranged in a sheet feed section of the sheet feeder.
In response to a recently increasing demand for using laser printers as printers for word processors and personal computers, an attempt has been made to meet a need for minimizing trouble of sheet shortage by setting a large number of frequently used sheets in the printers. However, such a conventional image forming apparatus requires replenishment of sheets in the course of printing a large number of sheets because the apparatus uses only sheet feed cassettes having a predetermined capacity.
To overcome the problem, sheet feeders capable of receiving not only sheet feed cassettes having a standard capacity but also sheet feed cassettes having a large capacity are proposed in, e.g., Unexamined Japanese Utility Model Publications Nos. 8954/1985 and 59264/1985. Unexamined Japanese Patent Publication No. 230164/1985 discloses an image recording apparatus that designs a sheet feed section as a single sheet feed unit and allows as many sheet feed units to be added to the apparatus body unit as are required.
However, the above-described apparatuses disclosed in Unexamined Japanese Utility Model Publications Nos. 8954/1985 and 59264/1985 have such a structure that a sheet feed cassette is installed to the bottom of the apparatus body. This makes the operation of installing and removing the sheet feed cassette more difficult than the type in which the sheet feed cassette is installed from the side of the apparatus body. In addition, when the sheet feed cassette is replaced with that having a large capacity, the apparatuses additionally require the operations of adjusting the height of a member supporting the apparatus body and replacing such member itself.
Further, an apparatus proposed in Unexamined Japanese Patent Publication No. 230164/1985 can use only sheet feed cassettes having a predetermined capacity (e.g., 250 sheets). Therefore, when a large number of sheets are printed, a new sheet feed cassette must be selected every time the sheets in a sheet feed cassette run out, making the operation troublesome. Still further, the apparatus requires a sheet feed section for each sheet feed cassette, making the apparatus not only mechanically complicated but also economically disadvantageous.